The structures of small protein motifs, including those of the dimerization domain of the yeast transcription factor GCN4, have proved difficult to solve using molecular replacement techniques. We are developing a technique for solving such structures with MAD data using selenomethionine-substituted peptides. We have mutated a single leucine in the core of the GCN4 dimerization domain and have seen a switch from dimer to trimer of the oligomerization state of the peptide. We have already collected data on the mutant without selenomethionine, but molecular replacement has yielded no solution and all metal derivatives have destroyed the crystals. In order to understand how the wild type core residues pack in the trimer, we must solve the structure of this mutant using MAD techniques.